1. Field of the Invention
The present invention relates to dielectric filters of the type including a plurality of dielectric resonators.
2. Related Art
A dielectric filter of the type referred to above is shown in FIG. 8 which illustrates an exemplary embodiment of such a dielectric filter. In FIG. 8, the dielectric filter includes a dielectric block A and a plurality of resonators provided in the dielectric block A in parallel to each other. Each resonator is formed by providing an inner conductor C on a corresponding wall surface of a through-hole B extending from a first end surface A1 of the dielectric block to a second end surface A2 of the dielectric block opposite the first end surface A1. An outer conductor D is provided on the circumferential surface (i.e., the side and end wall surfaces) of the dielectric block A except for the first end surface A1 of the dielectric block A. The inner conductor C provided on the wall surface of each through-hole B and the outer conductor D provided on the circumferential surface of the dielectric block A are connected with each other so that the second end of end wall surface A2 serves as a short circuit end surface. The first end or end wall surface A1 serves as an open end surface or open circuit end surface. A variety of different dielectric filters of a similar structure have been proposed, and these filters are typically employed as high-frequency band filters.
In the dielectric filter of FIG. 8, a pair of input-output terminals E1 and E2 are provided on a side surface A3 of the dielectric block A so that the input-output terminals E1 and E2 are located adjacent to the open end surface at respective positions corresponding to open ends of two of the resonators. Around the input-output terminals E1 and E2, insulating sections F1 and F2 are provided in such a manner that each insulating section isolates a corresponding one of the input-output terminals E1 and E2 from the outer conductor D formed on the side surface A3 of the dielectric block A. In order to lower production costs and increase productivity, the input-output terminals E1 and E2 are generally formed after the production of a plurality of the filter elements each including a plurality of dielectric resonators. More specifically, the plurality of filter elements are held together by means of a jig, and a silver paste is applied to the filter elements through screen printing, while areas corresponding to insulating sections F1 and F2 which define the terminals E1 and E2 are masked, to thereby form the conductor layer D. Therefore, the conductor layer D remains between the insulating sections F1 and F2.
In conventional dielectric filters such as that shown in FIG. 8, the outer conductor present between the insulating sections defining the pair of input-output terminals serves the function of preventing magnetic field coupling. However, this outer conductor also narrows the bandwidth of the filter. In the latter regard, the filtering characteristics of such a conventional filter are shown in FIG. 9.